The present invention relates to a nozzle for guiding molten metal, for example molten steel. More particularly, the invention relates to a so-called submerged entry nozzle, sometimes also known as a casting nozzle, used in the continuous casting process for producing steel. The invention also relates to a method of guiding molten metal using the nozzle.
In the continuous casting steelmaking process, molten steel from a ladle is poured into a large vessel known as a tundish. The tundish has one or more outlets through which the molten steel flows from the tundish into one or more respective moulds in which the molten steel cools and solidifies to form continuously cast solid lengths of the metal. A submerged entry nozzle, which has the general form of an elongate conduit (it generally has the appearance of a rigid pipe or tube) is located between the tundish and each mould, and guides molten steel flowing through it from the tundish to the mould.
The main functions of the ideal submerged entry nozzle are as follows. Firstly, the nozzle serves to prevent the molten steel from coming into contact with air as it flows from the tundish into the mould, since air would cause oxidation of the steel, which is undesirable. Secondly, it is highly desirable for the nozzle to introduce the molten steel into the mould in as smooth and non-turbulent a manner as possible, since turbulence in the mould causes the flux on the surface of the molten steel in the mould to become dragged down into the steel (known as xe2x80x9centrainmentxe2x80x9d), thereby generating impurities in the cast steel. Turbulence in the mould also disrupts the lubrication of the sides of the mould: one of the functions of the mould flux (apart from preventing the surface of the steel from coming into contact with air) is to lubricate the sides of the mould to prevent the steel adhering and solidifying to the mould and to prevent the consequent formation of surface defects in the cast steel. Minimizing the turbulence by means of the submerged entry nozzle is therefore important for this purpose also. Additionally, turbulence can cause stress on the mould itself, risking damage to the mould. Furthermore, turbulence in the mould can also cause uneven heat distribution in the mould, consequently causing uneven soldification of the steel and also causing variations in the quality and composition of the steel being cast. This latter problem also relates to a third main function of the submerged entry nozzle, which is to introduce the molten steel into the mould in an even manner, in order to achieve even solidified shell formation (the steel solidifies most quickly in the regions closest to the mould walls) and even quality and composition of the cast steel. A fourth function of an ideal submerged entry nozzle is to reduce or eliminate the occurrence of oscillations in the standing wave in the meniscus of steel in the mould. The introduction of molten steel into the mould generally creates a standing wave at the surface of the steel, and any irregularities or oscillations in the flow of the steel entering the mould can give rise to oscillations in the standing wave. Such oscillations can have a similar effect to turbulence in the mould, causing entrainment of mould flux into the steel being cast, disrupting the effective lubrication of the sides of the mould by the mould flux, and adversely affecting the heat distribution in the mould.
It will be appreciated that designing and manufacturing a submerged entry nozzle which performs all of the above functions as well as possible is an extremely challenging task. Not only must the nozzle be designed and manufactured to withstand the forces and temperatures associated with fast flowing molten steel, but the need for turbulence suppression combined with the need for even distribution of the molten steel in the mould create extremely complex problems in fluid dynamics.
U.S. Pat. No. 5,785,880 discloses nozzles in which the bottom outlet is divided into two ports by means of a flow divider. This design of nozzle is claimed to diffuse and decelerate the molten steel flow, and is also claimed to provide a generally uniform flow velocity distribution along the length and width of the outlet ports. This design of nozzle, it is claimed, has the consequence of reducing the size of oscillations in the standing wave in the meniscus of steel in the mould.
U.S. Pat. No. 5,944,261, which is a continuation-in-part of U.S. Pat. No. 5,785,880, discloses a submerged entry nozzle in which each of the two outlet ports is itself divided into two by means of a baffle, in such a way that the largest proportion of the molten steel flow throughput exits the nozzle via the two central ports. The particular shape and positioning of the baffles is claimed to diffuse the central streams and to cause recombination of the central streams with their respective outer streams upon exiting the nozzle. The consequence of this is claimed to be a reduction in the velocity of the molten steel exiting the nozzle, and a reduction in the turbulence created in the mould.
U.S. Pat. No. 6,027,051, which is a continuation-in-part of U.S. Pat. No. 5,944,261, discloses a variation on the design disclosed in U.S. Pat. No. 5,944,261, in which it is claimed that the effective discharge angle of the outer streams of molten steel varies depending upon the flow throughput. It is claimed that this has the effect of providing a smooth, quiescent, meniscus, over a range of flow throughputs.
One of the conclusions which will be most readily drawn from a consideration of the above patents is that seemingly minor, or even seemingly insignificant, changes in the design of a submerged entry nozzle can have dramatic effects upon the flow pattern of the molten steel flowing through, and out of, the nozzle. This is a consequence of the chaotic nature of fluid dynamics, in which small design changes to a conduit transporting a fluid can have profound effects upon the fluid flow pattern, and can even alter the nature of the fluid flow entirely.
The present invention seeks to provide a submerged entry nozzle which performs, as well as possible, the main functions of the ideal nozzle referred to above. The invention seeks to achieve this objective in a way which is entirely contrary to the teaching of the patents mentioned above, as will be explained below.
According to a first aspect, the invention provides a nozzle for guiding molten metal flowing from a vessel into a mould, the nozzle comprising a conduit which is elongate along an axis which is oriented substantially vertically during use, the nozzle having at least one upper inlet, at least two lower outlets which are inclined to the axis, and at least one lower outlet located generally axially between the inclined outlets, the minimum combined cross-sectional area of the inclined outlets being at least twice as great as the minimum combined cross-sectional area of the one or more generally axially located outlets.
The first aspect of the invention has the advantage that because the minimum combined cross-sectional area of the inclined outlets is at least twice as great as the minimum combined cross-sectional area of the one or more generally axially located outlets, the proportion of all of the molten metal flowing through the nozzle which flows out of the inclined outlets is generally significantly greater than the proportion which flows out of the generally axially located outlets. Preferably, at least 55% of the total molten metal flow exits the inclined exits and no more than 45% of the total molten metal flow exits the generally axially located outlets; more preferably, at least 60% of the total flow exits the inclined exits and no more than 40% of the total flow exits the generally axially located outlets. Because of the inclination to the vertical, of the inclined outlets, the downward vertical component of the velocity of the molten metal exiting such outlets is smaller than would be the case for vertically oriented outlets.
This has the effect of reducing the downward velocity of the majority of the metal entering the mould, and consequently reducing the turbulence created in the mould.
This is entirely contrary to U.S. Pat. No. 5,944,261 and U.S. Pat. No. 6,027,051, which teach that a greater proportion of the entire molten metal flow should flow through the lower (central) exit ports than through the upper (outer) exit ports, and in particular, 55-85% of the flow should exit the central ports and 15-45% of the flow should exit the outer ports.
The outlets which are inclined to the axis of the nozzle (i.e. the xe2x80x9couterxe2x80x9d or xe2x80x9csidexe2x80x9d outlets) may be substantially perpendicular to the nozzle axis, or they may be upwardly inclined (with the nozzle oriented as during use) with respect to the nozzle axis, for example. Preferably, however, the inclined outlets are downwardly inclined (with the nozzle oriented as during use) with respect to the nozzle axis. More preferably, the inclined outlets are downwardly inclined at an angle of 40xc2x0-60xc2x0 to the nozzle axis, even more preferably at an angle of 45xc2x0-55xc2x0 to the nozzle axis, for example approximately 50xc2x0 to the nozzle axis.
The or each outlet located generally axially between the inclined outlets preferably widens towards the exit of the outlet. This has the advantage of decreasing the velocity of the molten metal exiting the outlet, thereby decreasing the impact of the molten metal in the mould, and minimizing the turbulence created in the mould.
In some preferred embodiments of the invention, there are at least two (and preferably only two) outlets located generally axially between the inclined outlets, and preferably both (or all) such outlets widen towards their exit. For embodiments in which there are two such outlets, they are preferably located symmetrically on opposite sides of the nozzle axis.
The axis of the or each generally axially located outlet may be substantially coaxial with, or substantially parallel to, the axis of the nozzle. It is more preferred, however, at least for embodiments in which there is a plurality of generally axially located outlets, for the axis of each such outlet to be inclined with respect to the nozzle axis. Advantageously, the outlets may be downwardly inclined to the nozzle axis at an angle of 0xc2x0-30xc2x0 to the axis, more preferably at 5xc2x0-25xc2x0 to the axis, especially at 10xc2x0-20xc2x0 to the axis, for example at approximately 15xc2x0 to the axis.
Preferably the orientation and spacing of the inclined outlets and the generally axially located outlets is such that the molten metal streams exiting the generally axially located outlets during use do not combine with the molten metal streams exiting the inclined outlets (other than by the general mixing of all of the molten metal within the mould).
The minimum cross-sectional area of each outlet is as-measured perpendicular to the respective axis of the outlet, and the minimum combined cross-sectional area, respectively, of the inclined and the generally axially located outlets, is a combination of each of these measurements. As already mentioned, the minimum combined cross-sectional area of the inclined outlets is at least twice as great as the minimum combined cross-sectional area of the one or more generally axially located outlets. Preferably, the minimum combined cross-sectional area of the inclined outlets is at least three times as great, more preferably at least four times as great, as the minimum combined cross-sectional area of the one or more generally axially located outlets.
In preferred embodiments of the first aspect of the invention, at least the inclined outlets of the nozzle have a substantially constant cross-sectional area (perpendicular to their respective axes) along at least part of their length. In especially preferred embodiments, the inclined outlets have a restriction substantially at their innermost extremity, beyond which (in a direction towards their outermost extremity) the bore of each inclined outlet is wider. Beyond the restriction (where present), the bore of each inclined outlet is preferably substantially constant in cross-sectional area.
A second aspect of the invention provides a nozzle for guiding molten metal flowing from a vessel into a mould, the nozzle comprising a conduit which is elongate along an axis which is oriented substantially vertically during use, the nozzle having at least one upper inlet, and having at least two lower outlets which are inclined to the axis, the nozzle further comprising a receptacle located substantially axially between the inclined outlets, the receptacle having an upper opening and being defined by sidewalls which are substantially parallel and/or which converge towards the lower extremity of the nozzle, the receptacle receiving a proportion of the molten metal flowing through the nozzle in use prior to such molten metal exiting the nozzle.
The second aspect of the invention has the advantage that the receptacle located in the nozzle (which receives a proportion of the molten metal flowing through the nozzle prior to such molten metal exiting the nozzle) acts generally as a buffer which dampens oscillations or fluctuations in the flow rate of the molten metal flowing through the nozzle. This has the effect of reducing (or even, at least in some circumstances, substantially eliminating) the fluctuations or oscillations in the flow rate of the molten metal which exits the nozzle and enters the mould, thus reducing the likelihood of oscillations in the standing wave in the meniscus of steel in the mould. This consequently has the advantage that the likelihood of the entrainment of mould flux into the steel being cast, the disruption of the lubrication of the mould, and poor heat distribution in the mould, all of which can be caused or exacerbated by oscillations in the standing wave, are generally significantly reduced.
The dampening effect of the receptacle in the nozzle is created by the substantially axial location of the receptacle together with the shape of the receptacle, i.e. its substantially parallel and/or converging sidewalls. Because the receptacle is substantially axially located between the inclined outlets, it normally receives the full force of a significant proportion of the molten metal flowing through the nozzle, and because of its parallel and/or converging sidewalls, the receptacle generally absorbs a significant proportion of the momentum of the molten metal which it receives. In some embodiments of the invention, the receptacle may contain one or more outlets through which some of the molten metal flowing through the nozzle may exit the nozzle; in other embodiments the receptacle contains no such outlet and is entirely closed except for its upper opening. In either case, however, the effect of the receptacle is such that molten metal flowing out of the receptacle and into the inclined outlets does so in a generally consistent fashion, and such molten metal flowing out of the receptacle may also influence molten metal flowing directly into the inclined outlets from the elongate conduit of the nozzle, dampening variations in the flow rate of this part of the metal flow. Additionally, for those embodiments of the invention in which the receptacle itself contains one or more nozzle outlets, the molten metal exiting these outlets normally also has variations in its flow rate dampened. Both the concept and the practice of the nozzle receptacle according to the second aspect of the invention are entirely contrary to the teaching of U.S. Pat. No. 5,944,261 and U.S. Pat. No. 6,027,051, in which the baffles provided to divide the flow of liquid metal into outer and inner streams have diverging lower faces in order to diffuse the lower stream.
The receptacle is preferably defined by four sidewalls. Advantageously, at least two of the sidewalls converge towards the lower extremity of the nozzle, and more preferably all of the sidewalls so converge. Two opposite sidewalls of the receptacle are preferably provided by sidewalls of the nozzle itself; the other two sidewalls are preferably provided by structures located within the nozzle. Most preferably, the latter two sidewalls are provided by structures which also provide restrictions in the inclined outlets, as referred to above with respect to the first aspect of the invention.
In the most preferred embodiments of the invention, the first and second aspects of the invention are combined in one and the same nozzle.
Preferably, the receptacle is located above two generally axially located outlets. Two converging sidewalls of the receptacle are preferably provided by structures which define divisions between respective inclined outlets and generally axially located outlets.
The nozzle according to the invention is formed from refractory material. The refractory material preferably comprises ceramic material, for example a carbon-bonded ceramic material. Carbon-bonded ceramic materials are very well known in the art, and the skilled person will be able to select suitable materials for forming nozzles according to the present invention. The nozzle is preferably formed by isostatic pressing, which is the conventional technique for forming carbon-bonded ceramic articles.
A third aspect of the invention provides a method of guiding molten metal flowing from a vessel into a mould, utilizing a nozzle according to the first aspect of the invention.
A fourth aspect of the invention provides a method of guiding molten metal flowing from a vessel into a mould, utilizing a nozzle according to the second aspect of the invention.